Nonvolatile memories (NVM) retain stored information even if the power supply to the memory is switched off. There are different types of nonvolatile memories including Flash, read-only memory (ROM), one-time programmable (OTP), and multiple-time programmable (MTP) memories. Flash memory is the most versatile because it can be programmed and erased thousands of times with no degradation in the sense margin and data retention performance. Two types of NVMs are floating gate and SONOS.
A floating gate in a metal-oxide-semiconductor field-effect transistor (MOSFET) is used to store the charge. That is the floating gate is sandwiched in between the channel and the control gate. The method of injecting (programming of memory) and removing (erasing of memory) charge from the floating gate can be different and has led to different families of Flash memory. SONOS is a type of NVM closely related to Flash RAM, but is distinguished by the use of silicon nitride instead of polysilicon for the charge storage material. One SONOS memory cell is formed from a standard polysilicon N-channel MOSFET transistor with the addition of a small sliver of silicon nitride inserted inside the transistor's gate oxide. The sliver of nitride is non-conductive but contains a large number of charge trapping sites able to hold an electrostatic charge. The nitride layer is electrically isolated form the surrounding transistor, although charges stored on the nitride directly affect the conductivity of the underlying transistor channel. The SONOS charge storage layer may be composed of multiple dielectric layers. Hence, the “nitride” layer as described below can include several nitride layers and may also include nitrides with modified composition including addition of oxygen or silicon. Although it is called SONOS, it is not limited to ONO. The SONOS memory cell can have other structures like ONONO or other combinations. When the polysilicon control gate is biased positively, electrons from the transistor source and drain regions tunnel through the oxide layer and get trapped in the silicon nitride. This results in an energy barrier between the drain and the source, raising the threshold voltage VT (the gate-source voltage necessary for current to flow through the transistor). The electrons can be removed again by applying a negative bias on the control gate, either by de-trapping electrons or by tunneling holes into the charge trapping layer. SONOS uses lower programming voltages and higher program/erase cycle endurance than polysilicon-based flash memory
A key requirement for an NVM cell is reliability. The End-of-life (EOL) VT window is determined by the degradation caused by program/erase cycles (Endurance) and VT decay during storage (Data retention). Endurance is typically characterized by cycling a SONOS cell through the required number of program/erase cycles and measuring the shift of program voltage threshold (VTP) and erase voltage threshold (VTE). Retention is characterized by taking the SONOS cell through a fixed number of program/erase cycles and then measuring the change of VT (program or erase) with time at an elevated temperature. SONOS can be integrated at many of the technology nodes, such as 130 nm, 90 nm or 65 nm or the like. Endurance of SONOS degrades after about 10K cycles in scaled SONOS stacks such as used in 65 nm technology and other advanced technologies. This makes it difficult to achieve 100K/IM endurance cycle specs from EEPROM emulation and NVSRAM applications.